This invention pertains to a cathode-ray tube having an internal magnetic shield attached to a shadow-mask frame therein.
A shadow-mask cathode-ray tube typically has a magnetic shield to reduce the influence of magnetic fields on electron beam trajectories as a cathodoluminescent screen of the tube is scanned. In particular, the angles of incidence of the electron beams at every point on the shadow mask must not change significantly from the design values, or the beams will move away from their intended landing positions on the screen. The magnetic shield may be disposed either outside the tube as an external magnetic shield, or inside the tube as an internal magnetic shield.
The internal magnetic shield is usually made of 0.10 to 0.18 mm thick cold-rolled steel and is fastened to a shadow-mask frame by resilient clamping pins. The frame is supported by springs that engage mounting studs that extend inwardly from a glass rectangular faceplate panel of the tube. During tube fabrication, the internal magnetic shield is fastened to the frame prior to the steps of frit sealing a sidewall of the faceplate panel to a glass funnel of the cathode-ray tube. The internal magnetic shield is designed to fit into the funnel and to be as close to the funnel wall as possible. However, it should not touch the funnel to avoid any friction between the shield and a conductive anode coating on the inner surface of the glass funnel.
In order to be effective, a magnetic shield must be thoroughly demagnetized (degaussed) in position by subjecting the magnetic shield to the field from a degaussing coil energized by alternating current of progressively reduced amplitude. Degaussing is normally expressed in terms of ampere turns, and typically for an internal magnetic shield, would be in the order of 1500 A turns. This procedure effectively reorients magnetic domains in the shield and tends to leave it magnetized so as to nullify the field within the shield. The degaussing coil is typically built into the receiver, and the alternating current is automatically reduced from a high value to zero every time the receiver is turned on. This ensures against deterioration of color purity and white uniformity caused by changing magnetic field environments. After degaussing, the extent to which an electron beam strikes the cathodoluminescent screen closer to its intended landing position, measured in micrometers of residual misregister, is an indication of the effectiveness of degaussing recovery.
Using the same amount of degaussing current, the degaussing recovery for a cathode-ray tube with an additional external magnetic shield is usually better than that for a cathode-ray tube with an internal magnetic shield only. However, an external magnetic shield adds to the manufacturing cost. Consequently, in order to achieve a comparable degree of color purity using only an internal magnetic shield, it is necessary to improve its inherent degaussing recovery. The present invention provides for an internal magnetic shield showing a significant improvement in residual misregister after degaussing.